1. Field
This patent specification relates to the fabrication of semiconductor devices and, more particularly, to a method of determining the minimum amount of the projection exposure and the rate of etching used in a fabrication process, and a photomask used for the projection exposure.
2. Description
In the fabrication of semiconductor devices such as, for example, IC's and LSI's, optical projection printing is commonly utilized in the lithography process. During the projection printing, various patterns previously formed on a reticle or photomask are transferred by projection exposure to a photoresist film on a semiconductor wafer either with or without magnification. Latent images formed by the exposure are then developed to thereby form a photoresist pattern on the wafer.
In order to carry out the projection exposure accurately for ever miniaturizing patterns, it is of primary importance that the lithography technique form a photoresist pattern without deteriorating minute features of the pattern, and it is particularly important to determine accurately the minimum amount of the projection exposure E.sub.th for removing the photoresist film (Japanese Laid-Open Patent Application No. 9-96908/1987).
As an example of a method for determining an appropriate exposure amount, a method has been used which is described hereinbelow by reference to FIG. 5.
FIG. 5 is a top view illustrating a resist pattern formed on the surface of a wafer 50. For use in determining an amount of the projection exposure, a glass reticle is used with a photoresist film disposed thereon without a pattern. Projection exposures are carried out onto a plurality of portions consecutively on the photoresist film, with an exposure time different stepwise from one portion to the next. In the present example shown in FIG. 5, 28 exposures are made one onto each of the portions arranged in a 7 by 4 matrix as shown. The exposure time for each portion is also shown in FIG. 5, which increases from 0.1 second to 0.235 seconds in steps of 0.005 seconds. The photoresist film is subsequently developed, to result in a series of portions, as illustrated in FIG. 6, ranging from the portion from which the photoresist is completely removed, to the ones in which a part of the photoresist remains, to the ones in which only a part of the photoresist is removed, and to the one from which none of the photoresist appears to be removed. The minimum amount of the projection exposure E.sub.th is subsequently determined from the shortest exposure time obtained by observing the portions from which the photoresist is completely removed.
However, this method has a disadvantage in that a plurality of exposure steps have to be made, which is rather cumbersome and takes a relatively long time. Furthermore, in order to determine the E.sub.th value during the device manufacturing processes, a downtime has to be allocated (1) for switching the reticles, in which a manufacturing reticle is removed from a step-and-repeat photolithographic system (or stepper) and a glass test reticle is mounted instead there into and (2) for rewriting a test program by reloading a new set of parameters such as, for example, an initial exposure time and the amount as mentioned above. This has been another disadvantageous point.